The present invention relates to a printed circuit board trans-former comprising at least one primary winding and one secondary winding, each in the form of at least one layer of turns inside or at the surface of a printed circuit board, said transformer having a turn ratio of said primary winding to said secondary winding differing from 1:1 so as to achieve a step-up/down-operation.
Transformers of this type are used for transferring power and signals and for providing electric isolation and have for instance been proposed for isolated gate drive circuits for power MOS-FETs. It has turned out that such coreless printed circuit board transformers have desirable characteristics and can be employed for energy and digital signal transfer in low power applications, less than 100 W, in a high frequency range from a few hundred KHz to many MHz.
Furthermore, the invention is particularly directed to power transformers, i.e. transformers used for transfer of power, although the invention is neither restricted thereto.
A printed circuit board based transformer, often referred to as a planar transformer, is well known, through for example U.S. Pat. No. 6,501,364 B1. In such designs the use of a core material enhances the magnetic coupling between primary and secondary winding layers allowing an efficient power transfer even at relatively low frequencies. The size of the transformer will determine the total power capability and indeed the maximum frequency of operation. A higher switching frequency allows a smaller trans-former to be used for a given power level. Increasing frequency to be able to decrease the size of the transformer is limited by the increasing losses that occur in the core material. The demand for high efficiency has limited the maximum switching frequency used. Recent developments have increased the switching frequencies used, however, typical operation frequencies remain below 1 MHz. Removing the magnetic core of the trans-former leads to reduced coupling between primary and secondary winding layer turns and a loss in efficiency. A coreless transformer of this type is known through for instance EP 0 935 263 A2, but the transformer described therein is used for isolation purposes and uses a 1:1 turns ratio at high frequency to maintain coupling efficiency and does not provide for any step-up/down-operation.
The invention is particularly directed to transforming alternating voltages of a high frequency, such as in the range of 1-30 MHz. In order for the transformer to work efficiently at these frequencies and without a magnetic core it is necessary to obtain a very high coupling factor for the coupling of the magnetic fields of the primary winding and the secondary winding. This means that the local magnetic fields of the respective winding have to be in a good overlapping relationship, which may be obtained by arranging these windings close to each other. The distance between these windings may also, however, not be too close as, this would jeopardize the personal security and increase the capacitance and the resulting parasitic losses, a typical separation may be around 0.4 mm. The transformer according to the invention should provide for a step-up/down-operation, this significantly increases the complexity of the design of primary and secondary windings required to obtain a said high coupling factor necessary for enabling high operation frequencies.